This is a link. (I didn't link to anything. It's just for illustration.)
And the body tag for this page looks like this:
<body bgcolor="#FFFFFF" link="#669966" alink="#996666" vlink="#666699">
This is how you specify a colour:
#RRGGBB
i.e., two digits for red, two for green, two for blue... in hexadecimal.
Hexadecimal:
0 1 2 3 4 5 6 7 8 9 0 A B C D E F
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
...up to FF (=255).
All web browsers are supposed to display the following levels of each colour appropriately:
66, 33, 66, 99, CC, FF
Making a total of 216 guaranteed colours:
Most will display the rest reliably.